This invention relates to metal casting using a filter and is particularly suited to metal casting using direct pour methods, such as shown in U.S. Pat. Nos. 4,928,746 and 4,961,460 (incorporated by reference herein) although it is not limited to direct pour methods.
Conventionally molds such as sand molds or metal dies for casting molten metal have a mold cavity for producing the desired casting and a running system, usually comprising or consisting of a sprue, one or more runner bars and one or more in gates, and possibly one or more feeder cavities located above or at the side of the mold cavity. Because cast metals shrink during solidification, it is common practice to surround a feeder head with an exothermic and/or a thermally insulating feeder sleeve to retain the feeder head metal in molten state for as long as possible so as to improve the feeding effect and to enable the feeder head volume to be reduced to a minimum. (As used herein the terms "sprue" or "sprues" mean any passage or passages which is or are used to provide essentially the sole means of entry of molten metal into the mold cavity.)
The use of a well designed running system can reduce turbulence as well as ensuring that the mold cavity is filled satisfactorily but can entail the use of significantly more metal than is required for the production of the casting itself. It is not uncommon for the total weight of a casting running and feeder system to be up to about 50% of the total weight of a small metal casting.
The direct pour system and method was devised to overcome this problem and is described in European patent no. 0327226 and in U.S. Pat. Nos. 4,928,746 and 4,961,460. In those patents there is described a mold for metal casting having a mold cavity, a sprue communicating directly with the mold cavity, and--located in the sprue--a sleeve of refractory material having a filter fixed therein, the filter typically being a ceramic foam filter and located adjacent the lower end of the sleeve.
Also described in the aforesaid patents is a sleeve of refractory material having a filter fixed therein for use in a mold for casting metal as described in the immediately preceding paragraph, the filter being of a ceramic foam and located adjacent the lower end of the sleeve.
By utilizing the direct pour method of the aforesaid patents, the need to use a running system can be substantially or completely eliminated.
It has now been found that the invention of the aforesaid patents can be improved to provide a more flexible system that can be applied to a wider range of castings and is particularly suitable for the manufacture of castings of relatively high modulus, i.e. the ratio of volume to cooling surface, and low pour weight, and conversely for the manufacture of high volume castings of low modulus, although it will be appreciated that the invention is not limited to such castings.
Accordingly, in one aspect, the present invention provides a filter unit for use in metal casting, the unit comprising an adapter formed of refractory material, the adapter having one or more locations to receive a filter, so that the size of filter and adapter used may be selected to suit the metal pour weight and volume parameters of the casting, and a, filter positioned at said location or one of said locations.
In another aspect the invention provides an adapter as defined in the immediately preceding paragraph, preferably with a plurality of ledges.
In a yet further aspect the invention provides a sprue for metal casting, the sprue communicating directly or indirectly with a mold cavity, and located in the sprue a unit containing a filter as described in the two immediately preceding paragraphs.
Where the adapter has two or more locations to receive a filter, it is preferred that each location be designed to receive a filter of a size different from the other location(s).
The filter is preferably a foam filter, especially a ceramic foam filter, and will for convenience be more specifically described below with reference to ceramic foam filters. It will be appreciated, however, that it is not intended to be so limited, and other filters are also suitable as long as they effectively perform their filtering function in this environment.
The adapter may be used with or without a sleeve and in the former case may be shaped, e.g. tapered, to be a fit inside the correspondingly shaped sleeve, e.g. towards the lower end of the sleeve.
Where the adapter with its filter is used in a sprue as described above, the filter can be supported at all its sides, whereas in conventional practice it is not possible to support at all four sides due to the number of exits from beneath the filter leading to the mold cavity. To prevent risk of metal bypassing the filter, it is not possible to utilize the whole of the upper face of the filter in conventional practice.
Most conveniently, the location(s) for a filter is/are formed in the adapter as one or more internal ledges. Each ledge is preferably a continuous ledge extending completely around the internal perimeter of the adapter and provides a secure location for a filter as well as preventing metal bypass.
As indicated above, each ledge providing a single location for a filter is designed to accommodate a filter of size different from that to be accommodated by any other ledge. This can very conveniently be provided by forming the ledge or ledges as one or more steps on the internal perimeter of the adapter.
If desired, additional securing means may be provided at the filter locations, e.g. in the form of vertically extending and tapering projections on the inside perimeter of the adapter.
Where used, the sleeve may be designed to sit on top of the adapter, although it may be preferred to arrange for the adapter to sit inside the sleeve.
The outer surface of the adapter may be tapered in order that it may conveniently be fitted into a sprue or sleeve with the sprue or sleeve being correspondingly tapered to receive it. The direction of the taper will of course be determined by whether the unit is to be inserted into a preformed cavity from above or from below.
The unit is preferably located in a sprue so that its lower end is not in contact with the casting. This can be achieved by, for example, incorporating a ledge above the base of the sprue and seating the unit on the ledge.
The sleeve and the adapter may be made from the materials and in the manner described in the aforesaid patents. Thus they may be made from metals, ceramic materials, bonded particulate refractory materials such as silica sand and bonded refractory heat-insulating materials containing refractory fibers. For some applications the unit may also contain exothermic materials. It may be found advantageous in certain circumstances to make the adapter and sleeve of different materials.
Thus the adapter may be made in bonded refractory heat-insulating material and is made by dewatering onto a suitable former an aqueous slurry containing fibrous material and a binder and optionally particulate material, removing the product from the former, and then heating to remove water and to harden or cure the binder. Alternatively, the adapter may be made by other conventional techniques or means, e.g. shell molding or core shooting without the need for an oven to cure the binder.
For ease of manufacture the sleeve, where used, and adapter will usually be of substantially circular horizontal cross-section but that cross-section may be, for example, oval, oblong, square, or polygonal, to facilitate easier removal.
The ceramic foam filter may be made using well known methods of making ceramic foam, in which an organic foam, usually polyurethane foam, is impregnated with an aqueous slurry of ceramic material containing a binder, the impregnated foam is dried to remove water and the dried impregnated foam is fired to burn off the organic foam to produce a ceramic foam.
If desired the filter may be fixed af the chosen location in the adapter using an adhesive, the tapered projections referred to above, and/or any other convenient means.
As with the system described in the aforesaid patents, the present invention enables use of a mold with no running system apart from the sprue, but in addition to the mold cavity and the sprue the mold may also have one or more feeder cavities.
Additionally the present invention enables a wide range of casting modulus to be achieved from a single unit in which only the filter size need be changed. The unit enables larger filters to be used without the need for a sleeve, particularly where the use of a sleeve would be uneconomical because of its disproportionate size, and hence cost, necessitated by the size of the filter, since the adapter can be smaller and cheaper. This may, for example, be of particular value in the production of aluminum gravity diecastings.
The invention is particularly useful for the production of steel castings, especially of high modulus and low pour weight or, conversely, of low modulus and high pour weight, i.e. castings outside the conventional commensurate scope of filter and sleeve diameters. However, the invention can equally be used for other ferrous and non-ferrous castings, e.g. of aluminum, aluminum alloys, aluminum bronze, and magnesium, copper, zinc and their alloys.
The mold may be a sand mold prepared to conventional foundry practice or a permanent mold, such as a metal die, for producing castings by gravity diecasting or by low pressure diecasting.
When a casting requiring a feeder is produced using the unit of the invention, it is possible to locate the unit in the feeder cavity and to utilize the feeder as the sprue. In such applications it will be usual to use an adapter and/or sleeve which has exothermic and/or heat-insulating properties as well as being refractory in order to achieve satisfactory feeding of the casting.
A breaker core may be used, if desired, in the conventional manner or, if desired, the adapter of the invention may also be designed to act as a breaker core. Where a separate breaker core is used, it may be, for example, of the push fit type described in GB pat. no. 1597 832, the disclosure of which is hereby incorporated by reference herein Alternatively, where the adapter is designed to act as a breaker core, it may have a similar push fit design.
According to another aspect of the present invention a method of producing a metal casting is provided. The method comprises the steps of: (a) Providing a mold cavity having one or more sprues communicating with the mold cavity and providing substantially the sole entry of molten-metal for forming a metal casting into the mold cavity. (b) Locating an adapter of refractory material, capable of mounting a filter, at one or more sprues. (c) Optionally, locating the adapter in a sleeve, or a sleeve on the adapter. (d) Disposing a filter in contact with and supported by the adapter so that the filter is spaced from the surface of a metal casting to be produced. (e) Practicing steps (b)-(d) to select a size of filter and adapter to suit the metal pour weight and volume parameters of a metal casting to be produced. And, (f) pouring molten metal through the filter into the mold cavity to provide a smooth, substantially non-turbulent, flow of molten metal through the filter into the cavity, to produce a metal casting.
The method may also include the step of forming the mold of sand or metal, and providing one of the sprue and adapter with protrusions and engaging the other of the sprue and adapter with protrusions to hold the adapter in position in the sprue. Step (d) may be practiced by disposing a ceramic foam filter in contact with and supported by the adapter. Preferably, the adapter has a plurality of internal ledges each for receipt of a filter of a different size; and steps (d) and (e) are practiced to select the size of filter that corresponds to one of the ledges, and to place that filter into contact With, and mounted by, that ledge. Step (c) may be practiced to locate the sleeve on the adapter, or to locate the adapter in the sleeve, or the method may be practiced without a sleeve.
It is the primary object of the present invention to enhance a direct pour system and method by making it more flexible, including for use with a wider range of castings such as those of relatively high modulus and low pour weight, or for the manufacture of high volume castings of low modulus. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.